A central tire inflation system (CTIS) provides mobility enhancement through tire pressure adjustment and maintenance for on/off highway, commercial, non-commercial and tactical wheeled vehicles. An ideal CTIS provides fast inflation and deflation rates, enabling a vehicle to efficiently traverse surfaces having different hardness. For example, tires may be deflated, providing a larger wheel surface area for traversing relatively soft ground. System components are ideally closed from external environments to achieve long air seal life through reduction in moisture and contaminants and through enabling de-pressurization of seals when not actively adjusting tire pressures.
A typical CTIS includes a control interface providing manual and/or automatic adjustment of tire pressures. The control interface is communicatively connected to a compressor for providing air to the tires, and to a pneumatic control unit. The pneumatic control unit is responsive to the control interface and is communicatively connected between the compressor and a system of air transfer passages connected to and/or formed within the vehicle, such as in the frame and wheels. Rotary unions often communicatively connect the air transfer passages in the wheel to the air transfer passages in the remainder of the vehicle. A CTIS often also includes a wheel valve assembly, such as a wheel control valve assembly, disposed at each wheel to provide control of deflation and inflation of the respective tire.
Deflation rates are typically set by the primary controlling orifice in the wheel valve assembly. Faster deflation rates demand a larger cross-sectional flow area, such as at lower desired tire pressures where deflation rates are most affected. Closing forces required to seal against the control orifice of the wheel valve assembly increase with respect to increases in the effective area that the tire pressure works upon and/or with respect to increases in the desired maximum operating tire pressure. The closing force must be sufficient to overcome the desired maximum operating tire pressure, maintaining closure of the valve, while also being sufficient to react against resulting back pressure in the system, such as in the air transfer passages, due to the inherent downstream restriction of air flow through the lines and fittings of the air transfer passages, thus enabling the valve assembly to close. The minimum obtainable tire pressure control is therefore impacted by the maximum required closing force resulting from these variables.
Wheel valve assembly performance is also impacted by the effects of temperature and durability of components of the wheel valve assembly. Trapped volumes within the wheel valve assembly are subjected to the varying pressures due to varying operating temperatures of the wheel valve assembly. Closing forces must be capable of overcoming changes in pressure in these volumes. The wheel valve assembly must therefore be able to compensate for the high closing forces required at maximum desired inflation pressures and low temperatures, and remain open in order to deflate to the minimum desired inflation pressures at high temperatures at the desired deflation rate.